Hematopoietic cell transplantation (HCT) offers great promise for the treatment of a variety of malignant and non-malignant hematopoietic diseases, but graft-versus-host disease (GVHD) remains the major obstacle for success of HCT as it leads to high incidence of morbidity and mortality. Donor T cells that are included in the stem cell inoculum and recognize recipient alloantigens are the major cause of GVHD. When used as immunotherapy for hematopoietic malignances (e.g. leukemia), the therapeutic potential of allogeneic HCT relies on the graft-versus-leukemia (GVL) effect to eradicate residual tumor cells through immunologic mechanisms. T cell is a multi-potential precursor with defined antigen recognition specificity but substantial plasticity for differentiation into distinct lineages according to the signals encountered. Naive T helper cells (Th) can differentiate into four different subsets: Th1, Th2, Th17 and T regulatory cells, but the contributions of these activated T cell subsets to GVHD development and GVL effect remains unclear. Lacking the knowledge precludes us from selectively targeting the pathogenic subset or its associated cytokines for controlling GVHD while maintaining GVL effect. The goal of this project is to understand the contribution of Th1/Th17 differentiation, the cytokines that induce T-cell differentiation, and the cytokines produced by Th1/Th17 cells to GVHD development and GVL activity. The central hypothesis is that both the Th1 and Th17 subsets contribute to GVHD development but either lineage alone is sufficient to induce GVHD, and thus both lineages must to be blocked in order to control GVHD. This proposal will systematically and stringently addresses the contribution of Th1 and Th17 differentiation in GVHD by targeting lineage-specific transcription factor(s) for proof-of-concept (Aim 1). In parallel, more translational studies will be conducted to understand the role of Th17 priming and effector cytokine in GVHD and the reciprocal regulation of Th1/Th17 lineages in alloresponse in vivo (Aim 2). Because allogeneic HCT is primarily utilized to treat hematopoietic malignances, it is critically important to evaluate the contribution of different subset of T cells to GVL effect. We will evaluate the role of Th1/Th17 subsets in GVL effect along with GVHD development by using MHC-mismatched and -matched BMT models with leukemia or lymphoma. Furthermore, clinically applicable interventions will be used to block Th1/Th17 differentiation in these studies (Aim 3). The information learned from this research project will provide the rationale and means to regulate T-cell differentiation toward our ultimate goal of preventing GVHD while sparing GVL activity.